Hello. Sorry for not posting problem in one class but it would be just too much code and confusion.
The problem is in table I've created. It's dissapearing when i minimize <-> maximize my window. Reproducable in gwt hosted mode browser and in firefox3 (dunno about other browser, i didn't tested. I hope You could manage to include this patch in beta2!

ContentPanel north = new ContentPanel();
ContentPanel west = new ContentPanel();
ContentPanel center = new ContentPanel();
ContentPanel east = new ContentPanel();
ContentPanel south = new ContentPanel();